The present invention relates generally to printing plates for lithographic printing presses, and particularly to a method for modifying an image surface of a printing plate.
In lithographic offset printing press technology, ink is applied to an image surface of a printing plate. The image surface includes ink-loving and water-loving areas. Water adheres to the water-loving areas. The ink adheres to the ink-loving areas and is repelled from the water-loving areas. The ink which adheres to the ink-loving areas is transferred to a blanket and then to a sheet, web, etc., of paper or other material to produce the printed image.
Relatively complex processes are used to produce an image surface on a printing plate, i.e., to xe2x80x9cimagexe2x80x9d the plate. In the typical lithographic process, light, as from a laser or multiple lasers, is directed onto a light-sensitive substance on the surface of the printing plate. A laser imaging system is employed to scan the printing plate with the laser(s) so as to expose areas the light-sensitive substance in the pattern of a desired motif British Patent Document No. GB 2 284 684 describes a method of imaging a photosentsitive printing form using a coating sensitive to UV light using a mercury lamp and a system of mirrors and lenses.
According to one method for producing the image surface, the light-sensitive substance is a monomer which becomes polymerized upon exposure to light of the proper frequency, intensity and duration. Polymerization of the exposed areas occurs during the scanning process, so as to produce hardened ink-loving areas. The monomer at non-exposed areas is then washed away, leaving water-loving areas.
A problem with prior methods for producing an image surface is that most provide a relatively short plate life, i.e., a portion or all of the image surface wears out relatively quickly. There is no way for restoring even portions of the image surface without repeating the entire imaging process. Another problem with prior methods is that most provide a limited, if any, ability to correct small problemsxe2x80x94i.e., to remove or add an ink-loving layer to small and/or defined areas, for example to remove built-up ink and/or foreign matter from water-loving or ink-loving areas. It is known to use xe2x80x9cerase/writexe2x80x9d pens to manually correct dots, but this is difficult and only relatively large areas made up of multiple pixels may be corrected. As a result erase/write pens are not often used. Instead, entire plates are re-imaged to correct even small problems.
The present invention provides a method for modifying a surface of a printing plate. The method includes identifying a location of a pixel of an image at the surface, an ink being disposed at the location of the pixel; and irradiating the ink at the location of the pixel using a laser imaging system.
Identifying the location of the pixel may be performed using an optical analysis system on a printed product of the printing plate. Moreover, identifying the location of the pixel may be performed using the laser imaging system.
The laser imaging system may be an on-press laser imaging system useable to establish the image.
Irradiating the ink may be performed so as to harden the ink.
The method according to the present invention may further include adding a hardening agent to the ink prior to the irradiating, the hardening agent being curable by a frequency of an electromagnetic radiation emitted by the laser imaging system.
Irradiating the ink may be performed so as to harden the ink and produce a hardened ink surface useable for printing. The hardened ink surface may be capable of receiving additional ink for performing the printing.
The method according to the present invention may further include disposing additional ink at the location after at least one print run, and irradiating the additional ink so as to harden the additional ink and establish a renewed hardened ink surface useable for printing.
The ink may be disposed on a primary image surface of the image. The primary image surface may include a polymer substance.
The ink may be disposed on a printing substrate of the printing plate.
In an embodiment of the present invention, the ink may be hardened ink and irradiating the ink may be performed so as to loosen at least a portion of the ink. Irradiating the ink may be performed so as to ablate at least a portion of the ink.
The method according to the present invention may further include treating the ink with a fountain solution so as to remove at least a first portion of the ink.
The ink may be erroneously disposed at the location of the pixel.
The irradiating may be performed as part of a removal operation of at least a portion of the ink and the method according to the present invention may further include disposing an additional ink at the location of the pixel, and irradiating the additional ink at the location of the pixel using the laser imaging system so as to harden the additional ink.
The irradiating may be performed as part of a removal operation of at least a portion of the ink and the method according to the present invention may further include identifying a location of a second pixel of the image at the printing surface, disposing an additional ink at the location of the second pixel, and irradiating the additional ink at the location of the second pixel using the laser imaging system so as to harden the additional ink.
The irradiating may be performed with the printing plate disposed on a plate cylinder of a printing press.
The present invention also provides an image surface for a printing plate. The image surface includes a hardened ink layer at a location of a pixel of an image, the hardened ink layer being formed by an irradiation using a laser imaging system and being useable as a printing surface.
The present invention provides a renewable high-wear surface of hardened ink and a way of correcting errors by adding or removing a pixel. An on-press laser imaging system may be advantageously be used in performing the method according to the present invention.